Searching for signals of oxygenation in microbial reefs - Rosalie Tostevin

Searching for signals of oxygenation in microbial reefs
- Rosalie Tostevin
University of Capetown
Geochemical data suggest that atmospheric oxygen levels in the Proterozoic Eon were between 0.1-10% of the present day. While surface waters were oxygenated, the deep ocean remained largely anoxic-suboxic. The first animals appeared at the end of the Proterozoic, in the Ediacaran Period, pre-dating the rise to stable, modern-like atmospheric oxygen and fully ventilated oceans in the mid-Palaeozoic Era. However, the precise distribution, stability and amount of oxygen in Ediacaran oceans is poorly constrained. We investigate redox signals in the Nama Group, South Africa, an Ediacaran carbonate-ramp system deposited around 550 to 538 Ma. During this interval, global records suggest that oxygen levels fluctuated dramatically over ~1 myr timescales. Regional data suggest that anoxic waters impinged on the outer ramp, and animal fossils were confined to local oxygen oases. We targeted microbial reef systems, which grew in shallow, sunlit surface waters and were generated by a microbial community that likely included oxygenic cyanobacteria. They are therefore prime targets to search for signals of oxygenation. We micro-drilled microbial horizons, used sequential digestion techniques, and analysed rare earth elements + Y via solution and laser ablation ICP-MS. The samples preserve pristine primary seawater signals, but we found no evidence for Ce fractionation. This suggests that either oxygen levels were too low to trigger Ce oxidation or that microbial fabrics are not reliable recorders of ambient conditions. These data deepen a puzzling pattern of “missing” negative Ce anomalies throughout the Nama Group, despite clear evidence for fractionation, as indicated by abundant positive Ce anomalies recorded in non-microbial samples.

[@wcdeich4]

Could it be any oxygen produce was used by aerobic microbes? Just curious.

[@Denny_Boi]

It's likely. Aerobes were thought to have evolved in the last billion years, and would have followed the trends of oceanic and atmospheric oxygenation.

[@wcdeich4]

@@Denny_Boi Thank you for taking time to reply. I'm not an expert, just some random guy on the internet who is curious. If I understand correctly, recent developments in the study of prokaryotes suggest the very first eukaryotes could process oxygen. We discovered archaea are the prokaryotes most closely related to the nuclear DNA of eukaryotes. Then we found Lokiarchaeota, a group an anaerobic archaea that parasitize aerobic bacteria to gain aerobic abilities, like how our nucleus depends on mitochondria... And don't the genetic estimates for the origin of eukaryotes go back to about 2 billion years? I heard people suggest in parts of the ocean where light can reach, photosynthesizers may have produced small amounts of oxygen that was used up by aerobic bacteria & aerobic eukaryotes before it could diffuse very far. IDK if that is correct, but it's very interesting.